The present invention relates to a liquid crystal display (LCD) projection system and particularly relates to a projection system employing an active matrix LCD panel with sharp, high contrast ratios and minimum or non-existent secondary or ghost images on the display.
Optical projection systems, for example, overhead projectors, for displaying computer-generated images using liquid crystal displays are well known. See, for example, U.S. Pat. Nos. 4,154,007 and 4,763,993. In such systems, light from a light source passes through a focussing lens and through the liquid crystal display. Further optics associated with the projection system project the computer-generated image of the LCD display on an enlarged display surface. As well known, the optics of a standard liquid crystal display projection system include an LCD panel located between two protective light-transmissive glass panels. The glass panels are spaced from the LCD for protection purposes and also to provide spaces for flowing cooling air. Conventional LCD projection systems of this type typically use conventional uncoated glass panels in conjunction with a passive matrix LCD panel. As well known, passive matrix LCD panels have electronics which are typically multiplexed whereby each portion of the LCD is addressed for a fraction of the time for the entire field and is thus necessarily frequently readdressed. This type of energization of the crystals of the LCD not only provides a somewhat slow response time but, importantly, produces an image having a contrast ratio, projected or otherwise, which, while useful for certain purposes, is not as sharp or delineated as desired.
Typically, a passive matrix LCD panel in an LCD projection system is disposed and spaced between uncoated glass panels. Because passive matrix LCD panels have rather low contrast ratios, on the order of 10:1 and typically a maximum of about 20:1, reflections from optical components of the system are not noticeable and do not cause a problem in recognition of the generated images.
However, when using an LCD panel having a high contrast ratio, such as an active matrix LCD panel having light transmittance and contrast ratio characteristics much higher than passive matrix LCDs, the otherwise sharp, highly contrasted, and well-defined images typically have secondary or ghost images associated therewith. For example, when high contrast ratios on the order of 50:1 and greater, are obtained by using the active matrix LCD panels in the projection system, the image display has secondary reflections or ghosting images along the borders of the characters. Thus, where it has been attempted to employ an active matrix LCD panel in a projection system, the beneficial higher contrast ratios generated, as compared with those generated in passive matrix LCD panels, is degraded by these ghosting or secondary images within the system.
As well known, active matrix panels, for example, of the thin-film transistor type, maintain the signal charge within pixel electrodes. Thus, multiplexed transistors act as switches that allow the active matrix LCD to alternately block and pass light for extended periods corresponding to the scan time for the field, and therefore afford higher contrast ratios, for example, greater than 50:1 in comparison with contrast ratios of less than 20:1 for passive matrix displays.
According to the present invention, it has been discovered that the source of the secondary or ghost images, in an LCD projection system which employs an active matrix LCD panel, is light reflected from the optical components of the system and particularly reflected from the upper glass panel and back by the polarizer. It will be recalled that the optical path from the light source in a projection system of this type passes through a lens which focuses the light for transmission along an optical path non-perpendicular to a plane containing the LCD. More particularly, the light from the projector passes through the bottom glass panel, through the LCD panel and through the top glass panel at an angle other than 90.degree. relative to the LCD and its protective glass panels. It has been discovered that light is reflected from the top glass panel back onto the LCD, and is then reflected again from the LCD back to and through the top glass panel and that it is this light which causes secondary or ghost images to occur about the image characters, thereby generally degrading the contrast ratio. Additional further reflections of the reflected light also occur, compounding the problem. Because the light passing through the LCD panel is angled relative thereto, the reflections occur a slight distance away from the original image, thus producing the secondary or ghosting images in the display. In short, the secondary or ghost images are the result of light angled by the focussing lens in the projection system before it enters the LCD panel whereby reflections in the optical components of the LCD are inherent.
Having discovered the source of the problem associated with secondary or ghosting images in a projection system using an active matrix LCD panel, the present invention solves that problem by providing an attenuation of the reflected light sufficient to minimize or eliminate these secondary or ghost images. To accomplish this, the reflections from the top glass back to the LCD panel are reduced or eliminated while maintaining transmissivity. The source of the problem lies in the employment of conventional uncoated glass, such as soda lime glass, which reflects a substantial amount of light from its surface, such glass being desirable in these systems, however, because of its high degree of light transmittance. When used with the active LCD matrix panel, however, the secondary reflections from such glass are unacceptable. Efforts to reduce or eliminate the reflected-back light from the glass by using conventional acid-edged anti-glare glass have not proved satisfactory because of decreased light transmittance of the optical system employing that glass. Such anti-glare glass also distorted the resulting image.
In order to solve the problem, a thin-film optical coating has been found which, when applied to conventional glass, such as soda lime glass, alters the reflective properties of the glass to eliminate, minimize or attenuate the reflected light sufficiently to avoid the formation of secondary or ghost images on the projected surface. Moreover, the particular coating has a property which enhances and beneficially increases the transmissivity of light through the glass. In LCD projection systems, light transmittance is very important to image quality and any increase in transmittance is beneficial in enhancing image quality. Thus, by applying a thin-film anti-reflective, high light transmittance coating along the underside of the overlying glass panel, for example, of a type known as a High-Efficiency Anti-Reflective (HEA) coating, manufactured by Optical Coating Laboratory, Inc., 2789 North Point Parkway, Santa Rosa, Calif., it has been found that the secondary or ghosting images resulting in projection systems using an active matrix LCD panel are substantially minimized, eliminated or attenuated.
In a preferred embodiment according to the present invention, there is provided apparatus for displaying images on a surface comprising a light source for projecting light along an optical path toward the surface, liquid crystal means disposed in the optical path, means for generating liquid crystal energizing signals and coupled to the liquid crystal means for selectively energizing portions thereof to define an image for projection on the surface, a glass panel spaced above the liquid crystal means and in the optical path and anti-reflective means disposed in the optical path adjacent the underside of the glass panel to substantially preclude light from being reflected back toward the liquid crystal means, thereby substantially precluding secondary reflections from interfering with the images displayed on the surface.
Accordingly, it is a primary object of the present invention to provide a novel and improved liquid crystal display projection system employing an active matrix LCD panel to produce sharp, undistorted images having high contrast ratios on the order of 50:1 and greater, without degradation of the quality of the image by light reflected in the optical components of the LCD.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.